In recent years, as the market for liquid crystal TVs expands, there has been an increasing demand for liquid crystal displays, especially for color liquid crystal displays. In order to further popularize the color liquid crystal displays in the future, lowering of the manufacturing cost is demanded, and a low-cost color filter (CF) substrate is especially sought after. In response to this, as a method of forming CFs (color layers) of a CF substrate, application of an inkjet method has been considered because the amount of the material used can be reduced.
One example of a method of forming a CF substrate by the inkjet method is described below with reference to FIG. 6. FIG. 6 is a cross-sectional schematic view showing an ink application to the substrate using an inkjet printing device.
In FIG. 6, a glass substrate 50 for forming CFs includes, on a principal surface thereof, dividing walls called banks 51 (walls for preventing color mixture) formed in a grid pattern, and a plurality of pixel regions are partitioned by the banks 51. Here, description will be made using an example of a pixel region 60a in which a red (R) CF is formed, a pixel region 60b in which a green (G) CF is formed, and a pixel region 60c in which a blue (B) CF is formed.
Preprocessing is performed on the glass substrate 50 arranged as described above in order for the banks 51 to have water repellency and for the openings of the pixel regions 60a to 60c to have hydrophilicity for separately painting different ink materials that are being applied.
The inkjet printing device 40 is equipped with nozzles 41a to 41c for applying (discharging) ink of the respective colors of red (R), green (G), and blue (B), and are disposed so as to face the pixel regions 60a to 60c. Then, once the ink of the respective colors of red (R), green (G), and blue (B) is discharged from the nozzles 41a to 41c, ink droplets 43a to 43c fall in the direction of the arrow A, and spread to the respective pixel regions 60a to 60c. By this, ink layers 53a to 53c are formed on the glass substrate 50, and the ink layers 53a to 53c are hardened by a baking treatment to become CFs. Formation of CFs by such a method has a merit of low costs because photolithography is not used.
However, CFs that were formed by the above-mentioned method could cause light leakage because the film thickness does not become even. This tendency is especially prominent in the rectangular pixel regions. The reason for this will be described with reference to FIG. 7. FIG. 7(a) is a schematic plan view of the pixel regions 60a to 60c, FIG. 7(b) is a schematic cross-sectional view along the line A-B of FIG. 7(a), and FIG. 7(c) is a schematic cross-sectional view along the line C-D of FIG. 7(a).
The ink layers 53a to 53c are made of a liquid material. Therefore, due to the effects of surface tension that the ink itself has, and due to the effects of being repelled by the banks 51 having water repellency, the film thickness becomes smaller near the banks 51, and the film thickness becomes larger near the center of the pixel regions 60a to 60c. Because of this, as shown in FIGS. 7(a) and 7(b), the ink layers 53a to 53c are able to secure sufficient film thickness and the ink is applied well near the center of the pixel regions 60a to 60c, but as shown in FIGS. 7(a) and 7(c), sufficient film thickness cannot be secured near the peripheral area of the pixel regions 60a to 60c, and regions 55 with no ink applied could be generated depending on circumstances. Due to this, color density became uneven in the pixel regions 60a to 60c, and display quality of a displayed image was sometimes lowered. Additionally, in FIG. 7(c), an outline arrow shows how light leakage occurs.
In order to suppress such light leakage, as shown in FIG. 8(a), increasing the amount of the ink droplets to run on the banks 51 can be considered. However, if the amount of the ink droplets is increased, as shown in FIG. 8(b), a mixed region 54 where ink droplets are mixed between the adjacent pixels could be generated, and color mixture could occur.
Here, in order to cover the regions where the film thickness becomes smaller within the rectangular pixel regions, there has been suggested a method of chamfering the bank shape in the corner area of a pixel region to prevent significant light leakage from the four corners of the rectangular pixels and the like (for example, see Patent Document 1). However, although light leakage can be reduced by this method, the pixel regions become smaller. Therefore, there was room for further improvement in order to increase the aperture ratio.
Further, there has been suggested a method of generating power to planarize the cross-sectional shape of the ink by forming grooves and holes, which are for letting ink out, in the banks in an organic electroluminescence (EL) display devices (for example, see Patent Document 2). However, processes for creating grooves and holes are required in this method, and therefore, the process becomes cumbersome. Furthermore, because the banks also function as light shielding portions, light that is supposed to be blocked passes through by forming grooves, holes and the like, and therefore, if this method is applied to a liquid crystal display panel, display quality could be lowered because of malfunctions due to light shielding failure of TFTs (Thin Film Transistors), light leakage from the regions other than the liquid crystal driving regions, reflection of outside light from a metal wire formed in a TFT substrate, and the like.
Moreover, there has been suggested a method of obtaining CFs with even film thickness by performing a planarizing process on a surface of the applied ink (for example, see Patent Document 3). However, in this method, the steps are cumbersome because the planarizing process is performed on a surface of CFs by applying pressure using a pressure roller or the like, scraping off the surface by a blade or the like, cutting off the surface by a cutter or the like. There is also a concern that the surface of the CF substrate may deteriorate.
Further, there has been suggested a method of specifying the contact angle of the banks and the ink material to prevent an ink material applied to pixel regions from going over the banks to mix with different colors (for example, see Patent Document 4). With this method, by adding water repellency and/or hydrophilicity to banks by a plasma treatment or the like, the contact angle of the banks and the ink material can be specified, and by this, color mixture can be prevented. However, there was room for improvement in terms of preventing color mixture more easily.